Integrated circuits are typically manufactured on a semiconductor substrate, such as a silicon wafer. The silicon wafer is a thin circular plate of silicon of 150 or 200 millimeters in diameter and approximately 25 mils thick. A single wafer has numerous integrated circuit devices that are imprinted on the wafer and comprise a lattice of devices. Each device consists of numerous layers of circuitry and a collection of bonding pads. The bonding pads are typically small pads of three mils square which serve as connections to the device pin leads.
Prior to the packaging of integrated circuit devices into carriers, the devices are often tested to avoid packaging bad devices. The testing process involves connecting a probe card to a tester. The probe card has a collection of electrical contacts or pins that stand in for the normal pins and wire leads of a packaged device. The wafer is then positioned so that the pins on the probe card make contact with a given device""s bonding pads and the tester runs a battery of electrical tests on the device. A special machine, called a wafer prober, is used to position each device on the wafer with respect to the probe card. Each device on a wafer is tested in turn as a probe is driven by a stepper motor. High accuracy is required, because the bonding pads are small. If a probe card pin makes contact outside the pad area the device must be repositioned, slowing the testing process, and requiring operator intervention. Moreover, incorrect positioning can cause damage to the device since the probe card pins may break through the passivation layer of the device.
The primary purpose of wafer probing is to accurately position the wafer so that the device""s bonding pads make sufficient electrical contact with a probe card""s probe tips. Proper positioning requires that the prober accurately align a wafer to a specified axis relative to the motion axis of the prober motor. Typical methods of positioning a wafer in a tester involve taking an image of the die on a wafer using a video camera and determining a feature on the wafer which can be used to align an axis of the wafer with the motion axis of the probe motor.
Present known methods of aligning a wafer on a chuck often exhibit unreliable performance. Problems with these known methods may involve harsh lighting conditions which interfere with the operation of the camera or the presence of fine details in the image which produce incorrect reference points in low resolution vision processors.
It is therefore an intended advantage of the present invention to provide an accurate method of determining the rotation of an object in a vision processor.
It is a further intended advantage of the present invention to provide a angle correction for coarse alignment of semiconductor wafers in a probe tester through a method and apparatus which is insensitive to noise and environmental conditions.
A method of determining the rotation of an object in a vision processing system is disclosed. An image of the object is digitally captured. The digital image is filtered through an edge detection operation to enhance the edge information contained in the image. The filtered image is rotated through a series of incremental angles to produce a series of rotated images. Each rotated image is projected onto an x-axis and y-axis defined by pixel grid axes defined by the original image. The projection of the rotated images produces projected pixel counts formed by the summation of pixel value differences of the x-axis and y-axis on the projected image. For each rotated image, a score is computed. The score corresponds to the sum of the difference of gray-scale values for adjacent projected pixels. The scores for the projections of each rotated image are plotted on a graph of score versus angle. A curve is defined which includes the highest score and the neighboring next highest scores. The curve is interpolated to determine the peak score. The peak score corresponds to the scene angle of the object relative to the x-axis and y-axis defined by the original image.
In one embodiment of the present invention, the scene angle is used in a wafer probing system to provide angle correction for coarsely aligning a semiconductor wafer relative to a test probe assembly.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.